Batteries provide their rated power at 77.degree. F. At this temperature, the battery delivers proper power, and upon discharge, the battery will accept a charge from a charging source. At temperatures lower than 77.degree. F, the viscosity of the electrolyte increases, thereby lowering the discharge capacity. This lowering of capacity occurs rapidly at temperatures below 32.degree. F. Another problem which occurs at temperatures below 77.degree. F is that a battery will not recharge properly. The battery, when charged with a constant voltage, will appear to recharge (i.e. it will come up to the rated voltage) but will not accept very much energy from the charging source. This is called a "surface charge". After two or three discharge--recharge cycles, the battery will be almost completely discharged of energy and will not be able to accept significant recharging.
In aircraft, this becomes critical due to the small capacity of the battery which is limited by weight considerations and due to the fact that many systems depend on electrical power. Sometimes the battery is discharged by a balked start or by a mistake such as leaving the master switch on. In this case, the battery will be destroyed by freezing if it cannot be recharged. Aircraft differ from automobiles in this respect because the battery is usually installed in a location other than the engine compartment and does not benefit from the heat generated by the engine.